Accumulative effect of repeated lightning impulses on transformer insulation: mechanism analysis

During the decades-long operation, oil paper insulation in power transformers may suffer the effect of thousands of lightning impulses. Such interesting but rarely studied phenomenon, also known as accumulative effect, may cause irreversible damage to the oil paper insulation, or even premature failure. Therefore, in this work, the influence of repeated lightning impulses on the oil paper insulation, especially the mechanism behind this phenomenon, was experimentally investigated. We determined the accumulative breakdown characteristics of the oil paper insulation and observed the influence of the repeated lightning impulses on the dissolved gas in oil and the degree of polymerization. Specifically, the space charge distribution in the oil paper samples and the relationship between the interval time and accumulative breakdown characteristics were experimentally determined in the end. The test results indicate that the accumulative effect only exists in the cellulose paper rather than the transformer oil. Such effect was considered to be the consequence of electrical degradation and accumulation of space charge.